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socket

NAME

socket −
Open a TCP network connection

SYNOPSIS

socket
?options? host port

socket
−servercommand ?options?
port

DESCRIPTION

This command
opens a network socket and returns a channel identifier that
may be used in future invocations of commands like
read, puts and flush. At present only
the TCP network protocol is supported over IPv4 and IPv6;
future releases may include support for additional
protocols. The socket command may be used to open
either the client or server side of a connection, depending
on whether the −server switch is specified.

Note that the
default encoding for all sockets is the system
encoding, as returned by encoding system. Most of the
time, you will need to use chan configure to alter
this to something else, such as utf−8 (ideal
for communicating with other Tcl processes) or
iso8859−1 (useful for many network protocols,
especially the older ones).

CLIENT SOCKETS

If the
−server option is not specified, then the
client side of a connection is opened and the command
returns a channel identifier that can be used for both
reading and writing. Port and host specify a
port to connect to; there must be a server accepting
connections on this port. Port is an integer port
number (or service name, where supported and understood by
the host operating system) and host is either a
domain-style name such as www.tcl.tk or a numerical
IPv4 or IPv6 address such as 127.0.0.1 or
2001:DB8::1. Use localhost to refer to the
host on which the command is invoked.

The following
options may also be present before host to specify
additional information about the connection:
−myaddraddr

Addr gives the
domain-style name or numerical IP address of the client-side
network interface to use for the connection. This option may
be useful if the client machine has multiple network
interfaces. If the option is omitted then the client-side
interface will be chosen by the system software.

−myportport

Port specifies an
integer port number (or service name, where supported and
understood by the host operating system) to use for the
client’s side of the connection. If this option is
omitted, the client’s port number will be chosen at
random by the system software.

−async

This option will cause the client socket to be connected
asynchronously. This means that the socket will be created
immediately but may not yet be connected to the server, when
the call to socket returns.

When a
gets or flush is done on the socket before the
connection attempt succeeds or fails, if the socket is in
blocking mode, the operation will wait until the connection
is completed or fails. If the socket is in nonblocking mode
and a gets or flush is done on the socket
before the connection attempt succeeds or fails, the
operation returns immediately and fblocked on the
socket returns 1. Synchronous client sockets may be switched
(after they have connected) to operating in asynchronous
mode using:

chan
configurechan−blocking 0

See the chan
configure command for more details.

The Tcl event
loop should be running while an asynchronous connection is
in progress, because it may have to do several connection
attempts in the background. Running the event loop also
allows you to set up a writable channel event on the socket
to get notified when the asynchronous connection has
succeeded or failed. See the vwait and the
chan commands for more details on the event loop and
channel events.

The chan
configure option -connecting may be used to check
if the connect is still running. To verify a successful
connect, the option -error may be checked when
-connecting returned 0.

Operation
without the event queue requires at the moment calls to
chan configure to advance the internal state
machine.

SERVER SOCKETS

If the
−server option is specified then the new socket
will be a server that listens on the given port
(either an integer or a service name, where supported and
understood by the host operating system; if port is
zero, the operating system will allocate a free port to the
server socket which may be discovered by using chan
configure to read the −sockname option). If
the host supports both, IPv4 and IPv6, the socket will
listen on both address families. Tcl will automatically
accept connections to the given port. For each connection
Tcl will create a new channel that may be used to
communicate with the client. Tcl then invokes command
(properly a command prefix list, see the EXAMPLES
below) with three additional arguments: the name of the new
channel, the address, in network address notation, of the
client’s host, and the client’s port number.

The following
additional option may also be specified before port:
−myaddraddr

Addr gives the
domain-style name or numerical IP address of the server-side
network interface to use for the connection. This option may
be useful if the server machine has multiple network
interfaces. If the option is omitted then the server socket
is bound to the wildcard address so that it can accept
connections from any interface. If addr is a domain
name that resolves to multiple IP addresses that are
available on the local machine, the socket will listen on
all of them.

Server channels
cannot be used for input or output; their sole use is to
accept new client connections. The channels created for each
incoming client connection are opened for input and output.
Closing the server channel shuts down the server so that no
new connections will be accepted; however, existing
connections will be unaffected.

Server sockets
depend on the Tcl event mechanism to find out when new
connections are opened. If the application does not enter
the event loop, for example by invoking the vwait
command or calling the C procedure Tcl_DoOneEvent,
then no connections will be accepted.

If port
is specified as zero, the operating system will allocate an
unused port for use as a server socket. The port number
actually allocated may be retrieved from the created server
socket using the chan configure command to retrieve
the −sockname option as described below.

CONFIGURATION OPTIONS

The chan
configure command can be used to query several readonly
configuration options for socket channels:

−error

This option gets the current error status of the given
socket. This is useful when you need to determine if an
asynchronous connect operation succeeded. If there was an
error, the error message is returned. If there was no error,
an empty string is returned.

Note that the
error status is reset by the read operation; this mimics the
underlying getsockopt(SO_ERROR) call.

−sockname

For client sockets (including
the channels that get created when a client connects to a
server socket) this option returns a list of three elements,
the address, the host name and the port number for the
socket. If the host name cannot be computed, the second
element is identical to the address, the first element of
the list.

For server
sockets this option returns a list of a multiple of three
elements each group of which have the same meaning as
described above. The list contains more than one group when
the server socket was created without −myaddr
or with the argument to −myaddr being a domain
name that resolves multiple IP addresses that are local to
the invoking host.

−peername

This option is not supported by
server sockets. For client and accepted sockets, this option
returns a list of three elements; these are the address, the
host name and the port to which the peer socket is connected
or bound. If the host name cannot be computed, the second
element of the list is identical to the address, its first
element.

−connecting

This option is not supported by
server sockets. For client sockets, this option returns 1 if
an asyncroneous connect is still in progress, 0
otherwise.